Lesion assessment using six degree of freedom ultrasound thermography

ABSTRACT

A diagnostic device and method for assessing lesion formation by measuring temperature changes during endocardial ablation. Intracardiac echo catheter data is accurately mapped into a model maintained by a visualization, navigation, or mapping system using the position and orientation of the intracardiac echo catheter transducer within the model. For each point in the model, either a frequency shift or echo time shift is calculated from the intracardiac echo data to determine local temperature changes, and the local temperature changes are displayed within a rendering of the model for the user.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/581,401, filed 29 Dec. 2011, which is hereby incorporated byreference as though fully set forth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The present disclosure relates to monitoring of therapeutic procedures.In particular, the present disclosure relates to apparatus and methodsfor monitoring and displaying lesion formation during therapeuticprocedures, such as cardiac ablation procedures utilized in thetreatment of cardiac arrhythmia.

b. Background Art

It is well known that atrial fibrillation results from disorganizedelectrical activity in the heart muscle (the myocardium). The surgicalmaze procedure has been developed for treating atrial fibrillation, andinvolves the creation of a series of surgical incisions through theatrial myocardium in a preselected pattern so as to create conductivecorridors of viable tissue bounded by scar tissue.

As an alternative to the surgical incisions of the maze procedure,transmural ablations of the heart may be used. Such ablations may beperformed from within the chambers of the heart (endocardial ablation),using endovascular devices (e.g., catheters) introduced through arteriesor veins. Various ablation techniques may be used, including, but notlimited to, cryogenic ablation, radiofrequency ablation, laser ablation,ultrasonic ablation, and microwave ablation. The ablation devices areused to create elongated transmural lesions—that is, lesions extendingthrough a sufficient thickness of the myocardium to block electricalconduction—forming the boundaries of the conductive corridors in theatrial myocardium. Perhaps most advantageous about the use of transmuralablation rather than surgical incision is the ability to performablation procedures without first establishing cardiopulmonary bypass(CPB).

Ablation devices are commonly used in conjunction with diagnosticsystems that aid the practitioner in navigating, positioning andorienting the ablation device. These systems can provide a visualreference, such as a three dimensional model or two dimensional image,allowing the physician to more easily determine the orientation of theablation device relative to the target anatomy. Intracardiac echo (ICE)catheters are one commonly used diagnostic tool that provides a twodimensional image of both therapeutic catheters and cardiac anatomy.

It is desirable for the practitioner (e.g., the doctor orelectrophysiologist) to be able to monitor local temperature changes atthe ablation site to allow the practitioner to more readily judge theextent of lesion formation during ablation procedures.

BRIEF SUMMARY OF THE INVENTION

The present disclosure, in one embodiment, describes a diagnostic systemthat utilizes an ICE catheter having an ultrasound transducer used togenerate ultrasound echo data sets representing the reflected ultrasonicenergy received by the transducer. The system also utilizes avisualization, navigation, or mapping (VNM) system configured togenerate a model of the heart, track the position of the ultrasoundtransducer within the model, generate one or more temperature voxelswithin the model, and generate a two dimensional rendering of the model.

The VNM system is also configured to receive multiple ultrasound echodata sets and for each data set generate a mapped value by mapping atleast a portion of the ultrasound echo data set onto a voxel element.The system further includes a display device configured to display agraphical user interface, and an electronic control unit (ECU) incommunication with the intracardiac echo catheter, the VNM system, andthe display device. The ECU is configured to receive the ultrasound echodata sets, the two dimensional rendering, and one or more user inputsdirecting the control of system components. The ECU is also configuredto generate a graphic user interface containing the two dimensionalrendering of the model as well as other display and control components.

The VNM system of the present embodiment generates a temperature valueby comparing the mapped values from multiple ultrasound echo data sets.The system can determine temperature changes by determining a frequencyshift or echo time shift between the mapped values of differentultrasound echo data sets. The voxels of the model can have a displayvalue indicating the color in which the voxel is to be displayed in thetwo dimensional rendering, where the display value is representative ofthe temperature change for that voxel.

The ICE catheter of the system may also generate a temperature signalthat can be used by the VNM system to generate an absolute temperaturevalue when combined with the temperature change value. The ECU can alsoreceive a temperature threshold as a user input that can be compared tothe absolute temperature value, and the ECU can change the display valueof a voxel when the absolute temperature of that voxel exceeds thethreshold.

In another embodiment, the present disclosure describes a method ofmeasuring temperature changes at a tissue treatment site comprising thesteps of generating a model of the treatment area, locating at least oneICE catheter within the model, receiving an first ultrasound echo dataset from the ICE catheter, and mapping the first ultrasound echo dataset onto a plurality of voxel elements within the model. Then beginningtreatment of the tissue, receiving a second ultrasound echo data setfrom the ICE catheter, mapping the second ultrasound echo data set ontoa plurality of voxel elements within the model, and generating atemperature change value for each voxel element by comparing the mappedportion of the first ultrasound echo data set and the mapped portion ofthe second ultrasound data set. The method can determine the temperaturechange for each voxel by determining a frequency shift or an echo timeshift from the mapped portion of the two echo data sets.

The method may also include a step that generates a two dimensionalrendering of the model containing one or more of the voxel elementswhere the voxel elements are depicted using a color representative ofthe temperature change value. The method may also include the steps ofreceiving a temperature signal from the ICE catheter, and generating anabsolute temperature value for each voxel element from the temperaturechange value for a given voxel and the temperature signal.

Where the method includes generating an absolute temperature value, themethod may also include the step of generating a two dimensionalrendering of the model containing one or more voxel elements where thevoxel elements are depicted in a color representative of the absolutetemperature value for each voxel element.

In yet another embodiment, the present disclosure describes a diagnosticdevice comprising an ECU in communication with an ICE catheter, a VNMsystem, and a display device. The ECU of the diagnostic device beingconfigured to receive a plurality of ultrasound echo signals and acatheter position and orientation signal. The ECU is also configured togenerate a model of a body cavity and locate the ICE catheter within themodel using the catheter position and orientation signal. The modelgenerated by the ECU can contain one or more voxel elements having adisplay value and a data value. The ECU of this embodiment is configuredto generate a set of echo data elements from each received ultrasoundecho signal where each echo data element is mapped onto one voxelelement based on the catheter position and orientation signal. The ECUcan generate a first echo data set and store the echo data elements asthe data value of the corresponding voxel elements. The ECU can thengenerate a second set of echo data elements and map each echo dataelement from the second set onto a voxel. For each voxel having a dataelement from both data sets mapped onto it, the ECU can generate adisplay value representing a temperature change for the voxel using thedata value and the mapped second data element.

In this embodiment, the temperature change can be generated bydetermining a frequency shift or echo time shift between the data valueand the second data element for each voxel. The ECU can also beconfigured to generate a two dimensional rendering of the modelcontaining one or more voxel elements where the voxel elements aredepicted in a color representative of the display value of each voxel.

The ECU of this embodiment can also be configured to receive atemperature signal and generate an absolute temperature value from thedisplay value and the temperature signal. Where the ECU generates anabsolute temperature signal, the ECU can be configured to generate a twodimensional rendering of the model containing one or more voxelelements, the voxel elements being depicted in a color representative ofthe absolute temperature value. Where the ECU generates an absolutetemperature signal, the ECU may also be configured to receive a userinput indicating a temperature threshold, where the ECU will generate atwo dimensional rendering depicting the voxel elements having anabsolute temperature value less than the temperature threshold using acolor representative of the absolute temperature value and the voxelelements having an absolute temperature equal to or greater than thetemperature threshold using an alert color.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the operation of the system ofthe present disclosure prior to the start of a therapeutic treatment.

FIG. 2 is a block diagram illustrating the operation of the system ofthe present disclosure after the beginning of a therapeutic treatment.

FIG. 3 depicts an example of an intracardiac echo catheter suitable foruse in the system of the present disclosure.

FIG. 4 depicts an example of the distal end of an echo catheter suitablefor use in the system of the present disclosure.

FIG. 5 a is a schematic view of the ultrasound transducer illustratingthe plane of emitted ultrasonic energy.

FIG. 5 b is a cross sectional view of the ultrasound transducerillustrating the plane of emitted ultrasonic energy.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIGS. 1 and 2 areblock diagrams illustrating the relationship between the elements of thesystem of the present disclosure.

The present disclosure provides a diagnostic system 10 capable ofmonitoring lesion formation during ablation. The diagnostic systemincludes an ICE catheter 12 containing one or more electrodes 13, ananatomical visualization, navigation, and mapping system (“VNM system”)14, an electronic control unit (“ECU”) 16, and one or more displaydevices 18. The VNM system 14 is used to detect therapeutic anddiagnostic devices within the body and locate the devices within a model20 of the heart and surrounding vasculature. The VNM system 14 can trackdetected therapeutic and diagnostic devices as they move within theheart and update their locations in the model 20 such that the model 20is updated in substantially real time. The VNM system can create a twodimensional rendering 22 of the model 20 depicting the position andorientation (“P&O”) of the devices within the model 20, which may beused as part of a graphic user interface depicted on the display 18.When the VNM system 14 is used to display the P&O of an ICE catheter 12within the heart, the echo plane of the ICE catheter 12 can be mappedwithin the model 20. Displaying the P&O of the ICE catheter 12 and itsecho plane allows a physician to more easily navigate an ICE catheter 12and thereby create an ICE image of a desired anatomical feature, such asan ablation target site. During the ablation procedure, the echo planeof the ICE catheter 12 can be used to estimate lesion formation bydetecting temperature changes at the ablation site.

Referring now to FIGS. 3 and 4, which illustrate an example of an ICEcatheter for use in the system of the present disclosure.

ICE Catheter

The ICE catheter 12 may comprise a control handle 24, a long flexiblebody member 26, and a sensor array 28 containing an ultrasound sensor 30and a plurality of electrodes 13. The sensor array 28 is fixed to theend of the flexible body member 26 and may be inserted intravenously andnavigated to the heart by manipulating the length of the flexible bodymember 26 using the control handle 24.

As depicted in FIGS. 5 a and 5 b, the ultrasound transducer 30 emitsultrasonic energy pulses in a generally fan shaped echo plane 34, andreceives echo pulses when the ultrasonic energy is reflected back to thetransducer 30 by a scattering object intersected by a pulse in the echoplane 34. The transducer 30 produces ultrasound echo data 36 (shown inFIG. 2) representative of the received echo pulses that can be used tocreate an ICE image depicting objects located within the echo plane 34.The ultrasound echo data 36 can be unprocessed ultrasound transducersignals, or may be conditioned using one or more amplification andfiltering circuits, such as, by way of example, backscatter A-line data.

ICE images may be gray scale images with tissue structures, cathetersand other dense objects being displayed in white, while dark portions ofthe image tend to represent cavity space filled with fluid. The moreechogenic (e.g., the denser) a material is, the brighter itsrepresentation will be displayed in the image. The image can begenerated by the ICE catheter 12, or by the ECU 16 of the system afterreceiving the ultrasound echo data from the ultrasound transducer 30.For purposes of clarity and illustration only, the description belowwill be limited to an embodiment having the ICE image created by the ECU16.

Referring now back to FIGS. 1 and 2, the ICE image generated by ECU 16can be incorporated into a graphic user interface 38. The graphic userinterface 38 is displayed on one or more of the display devices 18 ofthe system 10. In an alternative embodiment, the ICE image can beincorporated into the graphic user interface 38 such that the ICE imageappears projected within the two dimensional rendering 22 of the model20 maintained by the VNM system 14. Display of the ICE image within therendering 22 of the model 20 allows a physician to more easily correlatefeatures within the ICE image to anatomical features in the model 20.

The plurality of electrodes 13 or other sensors configured to beresponsive to the VNM system 14 allow the system 10 to determine the P&Oof the ICE catheter 12 within the model 20. The ICE catheter 12 iselectrically coupled to the ECU 16 and may contain three or moreelectrodes 13 responsive to an electric field 40 (shown in FIGS. 1 and2) generated by the VNM system 14. The electrodes 13 being configured togenerate a position response signal 42 when positioned within anelectric field 40 generated by the VNM system 14. The position responsesignal 42 may be received directly by the VNM system 14 or may becommunicated to the VNM system 14 through the ECU 16. The electrodes 13can be positioned within the sensor array 28 such that the VNM system 14may determine the P&O of the ultrasound sensor 30 with six degrees offreedom in the model 20 maintained by the VNM system 14, therebyallowing the ICE image to be located within the model 20.

In an alternative embodiment, one or more electrodes 13 may be replacedwith magnetic sensors responsive to a magnetic field generated by theVNM system 14. An example of such an ICE catheter 12 is described incopending U.S. patent application Ser. No. 12/982,968 filed Dec. 31,2010 entitled “INTRACARDIAC IMAGING SYSTEM UTILIZING A MULTIPURPOSECATHETER,” which is hereby incorporated by reference in its entirety asthough fully set forth herein.

VNM System

The ECU 16 is electrically coupled to (i.e., via wires or wirelessly) tothe VNM system 14, which generates and maintains the model 20 of theheart and surrounding vasculature or other body structure. The VNMsystem 14 may further be configured to allow the user to identifyfeatures within the model 20 and include the location as well as otherinformation associated with the identified feature, such as anidentifying label. By way of example, identified features may includeablation lesion markers or anatomical features such as cardiac valves.

The VNM system 14 functionality may be provided as part of a largervisualization, navigation, or mapping system, for example, an ENSITEVELOCITY™ system running a version of ENSITE NAVX™ software commerciallyavailable from St. Jude Medical, Inc., and as also seen generally byreference to U.S. Pat. No. 7,263,397 entitled “METHOD AND APPARATUS FORCATHETER NAVIGATION AND LOCATION AND MAPPING IN THE HEART” to Hauck etal., owned by the common assignee of the present application, and herebyincorporated by reference in its entirety.

The VNM system 14 may comprise conventional apparatus known generally inthe art, for example, the ENSITE VELOCITY™ system described above orother known technologies for locating/navigating a catheter in space(and for visualization), including for example, the CARTO™ visualizationand location system of Biosense Webster, Inc., (e.g., as exemplified byU.S. Pat. No. 6,690,963 entitled “SYSTEM FOR DETERMINING THE LOCATIONAND ORIENTATION OF AN INVASIVE MEDICAL INSTRUMENT” hereby incorporatedby reference in its entirety), the AURORA® system of Northern DigitalInc., a magnetic field based localization system such as the gMPS systembased on technology from MediGuide Ltd. of Haifa, Israel and now ownedby St. Jude Medical, Inc. (e.g., as exemplified by U.S. Pat. Nos.7,386,339, 7,197,354 and 6,233,476, all of which are hereby incorporatedby reference in their entireties) or a hybrid magnetic field-impedancebased system, such as the CARTO 3™ visualization and location system ofBiosense Webster, Inc. (e.g., as exemplified by U.S. Pat. Nos.7,536,218, and 7,848,789 both of which are hereby incorporated byreference in its entirety).

Some of the localization, navigation and/or visualization systems mayinvolve providing a sensor for producing signals indicative of catheterlocation and/or orientation information, and may include, for exampleone or more electrodes 13 in the case of an impedance-based localizationsystem such as the ENSITE VELOCITY™ system running ENSITE NAVX™software, which electrodes may already exist in some instances, oralternatively, one or more coils (i.e., wire windings) configured todetect one or more characteristics of a low-strength magnetic field, forexample, in the case of a magnetic-field based localization system suchas the gMPS system using technology from MediGuide Ltd. described above.

Although the exemplary VNM systems 14 described above each maintain amodel 20 of the body cavity, acceptable alternative mapping devices forcreating a model of cardiac structures include magnetic resonanceimaging (MR) and x-ray computed tomography (CT).

While each of the electric-impedance, magnetic field, and hybridmagnetic field-impedance based systems disclosed above can act as theVNM system 14 and remain within the scope and spirit of the presentdisclosure, the VNM system 14 of the remaining discussion will beassumed to be an impedance based system for the purposes of clarity andillustration unless otherwise noted.

ECU

Referring now to FIGS. 1 and 2 depicting a block diagram of embodimentsof the present invention, the ECU 16 will be discussed. The ECU 16 mayinclude a programmed electronic controller having a processor 44 incommunication with a memory 46 or other computer readable media (memory)suitable for information storage. Relevant to the present disclosure,the ECU 16 is configured, among other things, to receive user input 48from one or more user input devices electrically connected to the system10 and to issue commands (i.e., display commands) to the display devices18 attached to the system 10 directing the depiction of the graphic userinterface 38. The ECU 16 may be configured to be in communication withthe ICE catheter 12 and the VNM system 14 to facilitate the creation ofa graphic user interface 38 containing at least an ICE image, a twodimensional rendering 22 of the model 20, or both. The communicationbetween the ICE catheter 12, the ECU 16, and the VNM system 14 may beaccomplished in an embodiment through a communications network (e.g., alocal area network or the internet) or a data bus.

It should be understood that although the VNM system 14, the ICEcatheter 12, and the ECU 16 are shown separately, integration of one ormore computing functions may result in a system including an ECU 16 onwhich may be run both (i) various control and image formation functionsof the ICE catheter 12 and (ii) the modeling and position trackingfunctionality of the VNM system 14. For purposes of clarity andillustration only, the description below will be limited to anembodiment having the modeling and position tracking functionality ofthe VNM system 14 separate from the ECU 16.

Frequency Shift Temperature Estimation

In addition to generating an ICE image, the ICE catheter 12 may be usedto estimate lesion formation during an ablation procedure. Bypositioning the ultrasound transducer 30 such that the echo plane 34intersects the ablation target site, changes in temperature at the sitecan be detected by measuring local frequency shifts or echo time shiftsin the reflected ultrasound echo data 36. Detected temperature changesmay be visualized using a plurality of temperature voxels 50 within themodel 20 at the ablation target site. The system 10 allows localtemperature changes to be detected despite the movement of the ICEcatheter, the target anatomy—i.e., beating of the patient's heart, or acombination of the two—by mapping received ultrasound echo data 36 intothe model 20. Mapping data into the model allows the ECU 16 to comparediscrete ultrasound echo data points from the same tissue structures todetermine a relative temperature change for each point. As the relativetemperature changes are determined, the display values for eachcorresponding voxel 50 can be changed to reflect the new temperaturechange thereby causing the two dimensional rendering 22 to be updatedaccordingly.

In one embodiment of the system 10 of the present disclosure, the localtemperature changes at the ablation site are determined using theultrasound echo data 36 by measuring frequency shifts in the ultrasoundbackscatter A-line data as described in detail in Noninvasive Estimationof Tissue Temperature Responsive to Heating Fields Using DiagnosticUltrasound, Seip, Ebbini, IEEE Transactions on Biomedical Engineering,Vol. 42, No. 8, August 1995, which is hereby incorporated by referencein its entirety. The frequency shift method computes a detailedautoregressive power spectrum from the A-line data generated by the ICEcatheter 12 to model detected frequency shifts as tissue temperaturechanges. The frequency shift depends on the effect of a temperaturechange on two variables, the first being the average scatter spacing din the tissue and the second being the speed of sound c in the tissue,which can be expressed as:

$\begin{matrix}{{{f_{k}(T)} = {{\frac{{kc}(T)}{2\;{d(T)}}\mspace{31mu} k} = 1}},2,3,\ldots\mspace{14mu},{\infty.}} & (1)\end{matrix}$where k is the harmonic of the fundamental frequency, c(T) is the speedof sound in the medium as a function of temperature T, and d(T) is theaverage scatter spacing as a function of temperature.

The average scatter spacing d of a tissue increases as the temperatureincreases and decreases as the temperature decreases. This increase anddecrease of d as a function of temperature is determined by the linearcoefficient of thermal expansion a of the tissue or medium in general.The average scatter spacing d as a function of temperature isapproximately given by:d=d ₀(1+αΔT)  (2)where a is the linear coefficient of thermal expansion of the medium andd₀ is the average scatter spacing at a baseline temperature T₀.

The speed of sound c increases with increasing temperature in mosttissues, including muscle and other tissues containing mostly water, butdecreases with increasing temperature in fatty tissues. Bydifferentiating (1) with respect to time T and using (2) within theresult yields an expression for approximating a frequency shift Df_(k)as function of temperature:

$\begin{matrix}{{\Delta\;{f_{k}(T)}} \approx {{\frac{k}{2\; d_{0}}\left\lbrack \frac{\partial{c(T)}}{\partial T} \middle| {}_{T = T_{0}}{{- \alpha}\; c_{0}} \right\rbrack}\Delta\; T}} & (3)\end{matrix}$where c₀ is the value of the speed of sound in the medium at thebaseline temperature of T₀. As can be seen in (3), a frequency shiftdetected in the ICE catheter 12 A-line data can be used to approximate atemperature change when the linear coefficient of thermal expansion aand the temperature dependence of the speed of sound in the medium

$\frac{\partial{c(T)}}{\partial T}$are known.

Eco Time-Shift Temperature Estimation

In another embodiment of the system of the present disclosure, the localtemperature changes at the ablation site are determined using theultrasound echo data 36 by measuring echo time-shifts of ultrasoundscatter locations in A-line data as described in detail inTwo-Dimensional Temperature Estimation Using Diagnostic Ultrasound,Simon, VanBaren, Ebbini, IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control, Vol. 45, No. 4, July 1998, whichis hereby incorporated by reference in its entirety.

The thermal dependence of ultrasound echo time-shift is related tochanges in the speed of sound in the medium and to thermal expansion ofthe medium. The former produces an apparent shift in scatter location,while the latter produces a physical shift. The observed time shift foran echo from a scatterer at an axial depth z can be described as:

$\begin{matrix}{{\Delta\;{t(z)}} = {{{t(z)} - {t\left( z_{0} \right)}} = {2{\int_{0}^{z}{\left\lbrack {\frac{1 + {{\alpha(d)}\Delta\;{T(d)}}}{c\left( {d,{T(d)}} \right)} - \frac{1}{c\left( {d,T_{0}} \right)}} \right\rbrack{\partial\ \mathbb{d}}}}}}} & (4)\end{matrix}$where α(d) is the linear coefficient of thermal expansion of the medium,at depth d, ΔT(d) is the change in temperature at depth d, and c(d,T(d))and c(d,T₀) are the speed of sound at depth d at temperature T and aninitial temperature T₀, respectively. Differentiating (4) andsubstituting for the case where the thermal dependence on the speed ofsound in tissue is approximately linear, as is the case with ablationtemperatures, yields an expression for the change in temperature as afunction of the time shift.

$\begin{matrix}{{{\Delta\;{T(z)}} = {\frac{c_{0}(z)}{2}\left( \frac{1}{{\alpha(z)} - {\beta(z)}} \right)\frac{\partial}{\partial z}\left( {\Delta\;{t(z)}} \right)}}{where}{{\beta(z)} = {\left. {\frac{1}{c_{0}(z)} \cdot \frac{\partial{c\left( {z,T} \right)}}{\partial T}} \middle| {}_{T = {T\; 0}}\mspace{14mu}{{and}\mspace{14mu}{c_{0}(z)}} \right. = {{c\left( {z,T_{0}} \right)}.}}}} & (5)\end{matrix}$When the speed of sound in the medium and the coefficient of thermalexpansion are invariant with respect to depth, then c₀(z)=c₀, α(z)=α,and β(z)=β.

The algorithm for estimating temperature in a two dimensional planerequires tracking the cumulative echo time-shift ∂t(z,x,T_(i)) at eachlocation and time T_(i), and then differentiating it along the axialdirection (z) and filtering both axial and lateral (x) directions. Thesymbol T_(i) represents the time at which each ith frame was acquired,not the echo time delays t. The temperature estimation algorithm can beaccomplished by the following steps:

-   -   1) Acquiring two dimensional echo data from the ICE catheter 12        prior to any heating of the target anatomy to establish a base        line temperature r(z,x,T₀), i=0;    -   2) Start heating, i.e., by beginning the ablation treatment at        the target site;    -   3) Acquiring another two dimensional echo data reading        r(z,x,T_(i)), i=i+1;    -   4) Estimating the incremental time-shift map;        ∂{circumflex over (t)} _(incr)(z,x,T _(i))={circumflex over        (t)}(z,x,T _(i))−{circumflex over (t)}(z,x,T _(i-1))  (6)        at time T_(i) using the current and previous frames r(z,x,T_(i))        and r(z,x,T_(i-1));    -   5) Computing the cumulative time-shift map;

$\begin{matrix}{{\partial{\hat{t}\left( {z,x,T_{i}} \right)}} = {\sum\limits_{k = 1}^{i}\;{\partial{{\hat{t}}_{incr}\left( {z,x,T_{k}} \right)}}}} & (7)\end{matrix}$

-   -   6) Differentiating the cumulative time-shift map ∂{circumflex        over (t)}(z,x,T_(i)) along the axial direction and filtering        along the axial and lateral directions, using a two dimensional        separable FIR filter;    -   7) Scaling the results of the sixth step by kc₀/2 to obtain the        temperature-change map estimates a ∂{circumflex over        (θ)}(z,x,T_(i)) at time T_(i); and    -   8) Returning to step three and repeating until all of the        desired two dimensional echo data has been acquired.

The incremental time shifts can be very small and are commonly less thanthe echo sampling period. In order to obtain accurate incrementaltime-shifts in the echo subsample range an auto-correlation techniquecan be utilized to allow the incremental time-shift to be estimated fromthe phase of the axial component of a 2-D complex auto-correlation oftwo subsequent frames of echo data 36. The first step in of theauto-correlation function is to compute the analytic signal of the echodata using an FIR Hilbert Transformer h(m). The 1-D discrete timeHilbert transform of the echo signal along the axial direction can beobtained by convolusion of the discrete time sampled echo data and theHilbert Transformer h(m), given by:{hacek over (r)}(m,n,s)=r(m,n,s)*h(m)  (8)where r(m, n, s) is the discrete-time sampled echo data, where m is theindex along the axial direction, n is the index along the lateraldirection, and s is the frame index (wall clock time). An analyticsignal can be obtained using (8) through:{circumflex over (r)}(m,n,s)=r(m,n,s)−j{hacek over (r)}(m,n,s).  (9)

When using an echo image consisting of M samples along the axialdirection and N samples along the lateral direction, the q-th lag, alongthe axial direction of the complex auto-correlation function at location(m, n) and time s is defined as:

$\begin{matrix}{{\hat{\gamma}\left( {m,{n;q},0} \right)} = {\sum\limits_{m^{\prime} = {{- \frac{M}{2}} = \square}}^{\frac{M}{2} - 1}\;{\sum\limits_{n^{\prime} = {- \frac{N}{2}}}^{\frac{N}{2} - 1}\;{{\hat{r}\left( {{m + m^{\prime}},{n + n^{\prime}},{s - 1}} \right)} \cdot {{\hat{r}}^{*}\left( {{m + m^{\prime} + q},{n + n^{\prime}},s} \right)}}}}} & (10)\end{matrix}$where the superscript * denotes complex conjugation, and M and N areassumed to be even numbers for simplicity. From (10) the incrementaltime-shifts smaller than the sampling period can be estimated from thephase of the auto-correlation function computed at lags q=−1, q=0 andq=1 using:

$\begin{matrix}{{{\Delta\;{\hat{t}\left( {m,n} \right)}} = {\frac{2\angle{\hat{\gamma}\left( {m,{n;0},0} \right)}}{{\angle{\hat{\gamma}\left( {m,{n;1},0} \right)}} - {\angle{\hat{\gamma}\left( {m,{n;{- 1}},0} \right)}}}t_{sp}}},} & (12)\end{matrix}$where t_(sp) is the time for one sampling period of the RF-echo, and ∠is the angle operator. Estimates of Δ{circumflex over (t)}(m,n) can betruncated to the range [−t_(sp),t_(sp)] to avoid outliers whentime-shift increments are smaller than t_(sp).

The frequency domain and echo time-shift methods have been previouslyused with in relatively static configurations where neither the targettissue nor the ultrasound transducer is subject to appreciable movementduring the temperature estimation procedure. When movement is introducedinto the environment it becomes difficult to map newly receivedultrasound echo data to allow an accurate comparison with existing echoultrasound data.

An accurate comparison between the two data sets is difficult becausewhen the ICE catheter and the target tissue move relative to one anotheror the orientation of the ICE catheter changes relative to the targettissue the ultrasound echo data from the new position or orientationcannot be directly compared to data from the prior position ororientation without having known spatial relationship between theearlier and later data. Without a system for tracking the ICE catheter'sposition in the heart there is no known relationship between thetemporally distinct ultrasound echo data sets, which makes it verydifficult to produce accurate temperature determinations when movementoccurs. This static configuration limitation has previously renderedthese temperature estimation techniques inappropriate for dynamicenvironments such as the atrial chambers of the heart or other activemuscle systems.

The system of the present disclosure allows these temperature estimationtechniques to be extended to dynamic environments through theinteraction of the ICE catheter and VNM system when gathering theultrasound echo data used in either temperature estimation algorithm.

The VNM system 14 enables temperature estimation in a dynamicenvironment by tracking the movements of the ICE catheter 12 within theheart, which allows the system to accurately map received ultrasoundecho data 36 into the model 20 of the VNM system 14. Using the P&O ofthe ICE catheter 12, determined by the VNM system 14, each frame ofreceived ultrasound echo data 36 can be mapped into the model 20 usingvoxel elements 50 to represent the tissue temperatures.

As features in the cardiac or other therapeutic environment move, suchas the contraction of the cardiac chamber or movement of the ICEcatheter 12 within the cardiac chamber, the ultrasound echo data 36 canbe associated with the then existing P&O of the ICE catheter 12 and fromthat point be accurately mapped into the model 20. Thus, the model 20and the ICE catheter's 12 known position in the model 20 at all timesprovides a known spatial relationship between any two temporallydistinct set of ultrasound echo data 36 received by the ICE catheter 12.When ultrasound echo data 36 from multiple echo frames are mapped intothe same voxel 50 the data 36 from previous echo frames can be used withthe newly mapped frame to generate an estimated temperature change usingone of the methods previously discussed.

Now referring back to FIGS. 1 and 2, to accurately estimate temperaturechanges attributable to a therapeutic procedure, a baseline orbackground ultrasound echo data set 36 for the target anatomy should bemapped into the model 20. The baseline echo data 36 can be gathered bythe user while using the ICE catheter 12 to aid in navigating thetherapeutic catheter into position, or when creating echo images of thetarget anatomy and surrounding tissue prior to ablation.

Once the baseline echo data 36 has been mapped into the voxels 50 withinthe model 20, data gathered during the therapeutic procedure can be usedto generate temperature change estimates associated with the procedure.As the echo data 36 is mapped into the voxels 50 within the model 20,the display of the voxels 50 can be changed so as to create a visualreference of temperature at the therapeutic site. One such visualreference is a heat map, where the base line temperature is depicted asa green or blue color and, as the estimated temperature change of avoxel rises, its display color progresses along the visible lightspectrum, with red representing the highest temperature change. Arelative temperature change computed by the ECU 16 may be combined withtemperature data from other temperature sensors present in the cardiacenvironment to generate an absolute temperature estimation for eachvoxel element 50.

Varying the display of the voxel elements 50 from a minimum baselinetemperature to a maximum temperature set by the desired therapeutictemperature allows a user to easily determine which portions of thetarget anatomy have reached the desired therapeutic temperature. In oneembodiment of the system 10 a voxel's 50 display color can be changed toone not normally within the range of heat map temperatures, such as aviolet or purple color, making it more distinct from the surroundingtissue.

Although multiple embodiments of this invention have been describedabove with a certain degree of particularity, those skilled in the artcould make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this invention. All directionalreferences (e.g., upper, lower, upward, downward, left, right, leftward,rightward, top, bottom, above, below, vertical, horizontal, clockwise,and counterclockwise) are only used for identification purposes to aidthe reader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not limiting. Changes in detail or structure may be made withoutdeparting from the spirit of the invention as defined in the appendedclaims.

What is claimed is:
 1. A diagnostic system comprising: an intracardiacecho catheter having an ultrasound transducer configured to generate afirst ultrasound echo data set and a second ultrasound echo data setrepresentative of the received ultrasonic energy; a visualization,navigation, or mapping (VNM) system configured to generate a model of abody cavity, track the position of the ultrasound transducer within themodel, generate one or more voxel elements within the model, andgenerate a two dimensional rendering of the model, the VNM system beingfurther configured to receive the first ultrasound echo data set andgenerate a first mapped value by mapping at least a portion of the firstultrasound echo data set onto the one or more voxel elements, the VNMsystem being further configured to receive the second ultrasound echodata set and generate a second mapped value by mapping at least aportion of the second ultrasound data set onto the one or more voxelelements; a display device configured to display a graphical userinterface; an electronic control unit (ECU) in communication with theintracardiac echo catheter, the VNM system, and the display device andbeing configured to receive the first ultrasound echo data set, thesecond ultrasound echo data set, the two dimensional rendering, and oneor more user inputs directing the control of one or more of theintracardiac catheter and the VNM system, the ECU being furtherconfigured to generate a graphic user interface containing at least thetwo dimensional rendering; wherein the VNM system compares the firstmapped value with the second mapped value to generate a temperaturechange value.
 2. The diagnostic system of claim 1 wherein thetemperature change value is generated by determining a frequency shiftbetween the first mapped value and the second mapped value.
 3. Thediagnostic system of claim 1 wherein the temperature change value isgenerated by determining an echo time shift between the first mappedvalue and the second mapped value.
 4. The diagnostic system of claim 1,each of the one or more voxel elements having a display value, thedisplay value indicating the color in which the voxel is to be displayedwithin the two dimensional rendering, the display value beingrepresentative of the temperature change value.
 5. The diagnostic systemof claim 4, the intracardiac echo catheter being further configured togenerate a temperature signal and the VNM system being furtherconfigured to generate an absolute temperature value from thetemperature change value and the temperature signal, wherein the displayvalue is representative of the absolute temperature value.
 6. Thediagnostic system of claim 5, wherein the ECU receives a temperaturethreshold value as a user input and the VNM system changes the displayvalue if the absolute temperature value meets or exceeds the temperaturethreshold value.
 7. The diagnostic system of claim 1, the intracardiacecho catheter being further configured to generate a temperature signaland the VNM system being further configured to generate an absolutetemperature value from the temperature change value and the temperaturesignal.
 8. A method of measuring temperature changes at a tissuetreatment site comprising the steps of: generating a model of thetreatment area; locating at least one intracardiac echo catheter withinthe model; receiving a first ultrasound echo data set from theintracardiac echo catheter; mapping the first ultrasound echo data setonto one or more voxel elements within the model of the treatment area;beginning treatment of the tissue; receiving a second ultrasound echodata set from the intracardiac echo catheter; mapping the secondultrasound echo data set onto one or more voxel elements within themodel of the treatment area; and generating a temperature change valuefor each of the one or more voxel element by comparing the mappedportion of the first ultrasound echo data set and the mapped portion ofthe second ultrasound echo data set.
 9. The method of claim 8 whereinthe temperature change value for each voxel element is generated bydetermining a frequency shift between the mapped portion of the firstultrasound echo data set and the mapped portion of the second ultrasoundecho data set.
 10. The method of claim 8 wherein the temperature changevalue for each of the one or more voxel elements is generated bydetermining an echo time shift between the mapped portion of the firstultrasound echo data set and the mapped portion of the second ultrasoundecho data set.
 11. The method of claim 8 further comprising the step ofgenerating a two dimensional rendering of the model containing at leastone of the one or more voxel elements, the one or more voxel elementsbeing depicted in a color representative of the temperature change valuefor each voxel element.
 12. The method of claim 8 further comprising thesteps of: receiving a temperature signal from the intracardiac echocatheter; and generating an absolute temperature value for each of theone or more voxel elements from the temperature change value for each ofthe one or more voxel elements and the temperature signal.
 13. Themethod of claim 12 further comprising the step of generating a twodimensional rendering of the model containing at least one of the one ormore voxel elements, each voxel element contained in the two dimensionalrendering being depicted in a color representative of the absolutetemperature value for that voxel element.
 14. A diagnostic devicecomprising: an electronic control unit (ECU) in communication with anintracardiac echo catheter, a visualization, navigation, or mapping(VNM) system, and a display device, the ECU being configured to receivea plurality of ultrasound echo signals and a catheter position andorientation signal, the ECU being further configured to generate a modelof a body cavity and locate the intracardiac echo catheter within themodel using the catheter position and orientation signal, the modelbeing configured to include one or more voxel elements having a displayvalue and a data value, the ECU being further configured to generate aset of echo data elements from each received ultrasound echo signal,each echo data element being mapped onto one of the one or more voxelelements based on the catheter position and orientation signal, whereinthe ECU generates a first set of echo data elements, maps each elementfrom the first set of echo data elements onto a mapped voxel elementfrom the one or more voxel elements, and stores the echo data element asthe data value of the mapped voxel element, thereby creating a set ofmapped voxel elements, the ECU then generates a second set of echo dataelements, maps one or more elements from the second set of echo dataelements onto at least one voxel element from the set of mapped voxelelements, and for each mapped voxel element on which an echo dataelement from the second set of data elements is mapped, the ECUgenerates a display value representing a temperature change for thevoxel element using the data value and the second data element.
 15. Thediagnostic device of claim 14 wherein the display value is generated bydetermining a frequency shift between the data value of the voxelelement and the second data element.
 16. The diagnostic device of claim14 wherein the display value is generated by determining an echo timeshift between the data value of the voxel element and the second dataelement.
 17. The diagnostic device of claim 14, the ECU being furtherconfigured to generate a two dimensional rendering of the modelcontaining one or more voxel elements, the one or more voxel elementsbeing depicted in a color representative of the display value of each ofthe one or more voxel elements.
 18. The diagnostic device of claim 14,the ECU being further configured to receive a temperature signal andgenerate an absolute temperature value from the display value and thetemperature signal.
 19. The diagnostic device of claim 18, the ECU beingfurther configured to generate a two dimensional rendering of the modelcontaining one or more voxel elements, the one or more voxel elementsbeing depicted in a color representative of the absolute temperaturevalue.
 20. The diagnostic device of claim 19, the ECU being furtherconfigured to receive a user input indicating a temperature threshold,wherein the two dimensional rendering depicts the one or more voxelelements that have an absolute temperature value less than thetemperature threshold using a color representative of the absolutetemperature value of the one or more voxel elements and the one or morevoxel elements having an absolute temperature equal to or greater thanthe temperature threshold using an alert color.